The present invention relates generally to data communications within a computer. In particular, the present invention relates to the use of a slack register for time critical data flow control.
In a PCI transaction, a sequence of data items may be transferred in a pipelined fashion between an initiator and a target. The sequence of data items may be transferred either from a host bus buffer (which may be the initiator or the target) to a PCI agent (which is the target or the initiator, respectively) or from a PCI agent to the host bus buffer. For example, a PCI agent could initiate (as the initiator) a data read from the host bus (i.e., the target in this case).
The data items are sequentially clocked between the initiator and the target (e.g., between the initiator buffer and target buffer). The target and initiator each have an associated flow control signal (TRDY# and IRDY#, respectively) that is asserted when the target/initiator is ready for a data item to be transferred. For example, when an initiator initiates a read from a target, the TRDY# signal is asserted when the target is ready to send a data item to the initiator, and correspondingly the initiator asserts an IRDY# when it is ready to receive the data item. Thus, a data item will not be transferred in response to a clock trigger unless both the TRDY# and IRDY# signals are asserted during the clock trigger.
FIG. 1A shows a sequential storage device 10, such as a first-in, first-out (xe2x80x9cFIFOxe2x80x9d) buffer or random access memory (xe2x80x9cRAMxe2x80x9d) that serves as a data buffer for a target (e.g., the host bus). The device 10 includes a clock input 11, an active-low chip enable input 13, and a data output 15. Also shown is a receiver (e.g., an initiator) 12 having a flow control (e.g., IRDY#) output 14, a receiver clock input 16, and a receiver data input 18 for reading data received from the sequential storage device 10. The data output 15 of the sequential storage device 10 is in communication with the receiver data input 18 of receiver 12. A clock signal 17 is connected to storage clock input 11 and receiver clock input 16. In addition, an IRDY# signal 19 is outputted from flow control output 14 and connected to storage chip enable input 13. The clock signal 17 is a 66 MHZ PCI clock. The IRDY# signal 19 is a flow control signal for controlling the flow of data into the receiver (in this case, initiator) 12.
FIG. 1B shows a timing diagram for an illustrative initiator read operation for the sequential storage device 10 of FIG. 1A. The timing diagram shows a clock signal 17, an IRDY# signal 19, output data 215, and read data 212 which is data read by receiver (initiator) 12. If the IRDY# signal is asserted concurrent with a clock trigger, the clock trigger causes: (1) the initiator to read the current data item at data output 15 and (2) the sequential storage device 10 to clock the next data item out to data output 15. If the IRDY# signal is deasserted concurrent with a clock trigger, the initiator does not read the current data item at data output 15 in response to the clock trigger. (It may be assumed for purposes of this illustration that any other necessary control signals, e.g., TRDY#, for transferring data are asserted.)
On one hand, with a 33 MHZ PCI bus, the ERDY# and TRDY# signals provide at least a 6 or 7 nanoseconds (xe2x80x9cnsec.xe2x80x9d) set-up time prior to a clock trigger. This set-up time is predictably and consistently sufficient for enabling and disabling the sequential storage device 10. On the other hand, however, the 66 MHZ PCI bus specification (as shown in FIG. 1B) guarantees only a 3 nsec. set-up time for TRDY# and RDY# prior to a clock trigger. Unfortunately, 3 nsec. may not be sufficient for consistently enabling and disabling (through the chip enable input 13) the sequential storage device 10. Thus, as is shown in FIG. 1B, the deassertion (i.e., the IRDY# signal switching from a low state to a high state) of IRDY# during cycle 3 fails to meet the set-up time necessary to ensure that the sequential storage device 10 will not clock out the next data item (Data 4). Thus, data output from sequential storage device 10 during cycle 4 and thereafter is indeterminate. In addition, this deassertion of IRDY# indicates that the initiator will not read the current data item (Data 3) at the data output 15 in response to the next clock trigger (clock trigger 4). During cycle 5, the IRDY# signal is once more asserted indicating that the initiator will read the current data item (indeterminate data) in response to the following clock trigger (clock trigger 6), which is the first clock trigger subsequent to the assertion of IRDY#. Consequently, as shown in FIG. 1B, the overflow data item (i.e., the data item that was clocked out following the deassertion of the IRDY# signal) in this case Data 3, may be lost because the initiator failed to read it in response to the fourth clock trigger, which nonetheless may have clocked out the next data item. Also, the sequential storage device 10 may be out of synch with data transfers causing all subsequent transfers to be indeterminate. This loss of the overflow data item occurs regardless of whether the assertion of IRDY# in cycle 5 allows sufficient time to enable the sequential storage device 10 for the following clock trigger (clock trigger 6).
Accordingly, what is needed is a method and apparatus for transferring a sequence of data items with critically small flow control set-up times from a first sequential storage device to a receiver, such as an initiator buffer. In particular, what is needed is a way to transfer a sequence of data items between an initiator and a target (or vice versa) over a PCI bus when the flow control signals may not provide sufficient set-up times for disabling the source sequential storage device from overflowing an overflow data item in response to a clock trigger following deassertion of a flow control signal.
One embodiment of the present invention provides a circuit for transferring a sequence of data out of a sequential storage device, which is controlled by a flow control signal that may have an inadequate set-up and hold time. The sequential storage device has a data output to clock out data in response to a flow control signal being asserted. The sequential storage device terminates this clocking out of the data in response to the flow control signal being deasserted. However, overflow or indeterminate data may be relinquished by the sequential storage device and clocked out of the data output even after the flow control signal has been deasserted. The circuit includes a slack register that is connected to the data output of the sequential storage device. The slack register stores the overflow data until the flow control signal is again asserted.